The present invention relates to a method of foreseeing and/or controlling tread wear in a pneumatic tire and a related pneumatic tire suitably arranged for use of such a verification method. More particularly, the invention relates to a method of foreseeing and/or controlling tread wear in a tire while leaving substantially unchanged the integrity of the tire.
It is known that a pneumatic tire generally consists of a carcass of toric form, having a crown on which is a disposed tread band of elastomeric material constituting the ground-contacting portion of the tire running on a road.
The tread band is generally comprised of a raised pattern mainly consisting of a plurality of land portions in the form of ribs and/or blocks distributed according to various configurations, defined and separated from each other by a plurality of grooves directed longitudinally and/or transversely of the tire, the depth of said grooves defining the height of said land portions.
It is known that during tire use the tread is subjected to wear, which wear can be classified according to two distinct types, that is one of (a) an uneven type identified by the appearance of localized early wear marks at some particular areas, the land edges for example where abrasion is more pronounced than at other areas, and (b) one of an even type that goes on at a given velocity until the complete disappearance of the tread pattern.
Uneven wearing is particularly dangerous because it greatly affects the tire behavior on driving, giving rise to vibrations and difficulties in keeping the desired path; in addition, since this wearing reveals itself in the form of localized spots even of great width, it can involve extra costs due to the impossibility of using a tire which, taking into account the remaining tread portion, could otherwise still be used, since the residual height of the pattern lands are still prominent.
Even wearing affects the tire behavior on the road to a lesser degree and in particular its effect consists of progressively reducing the tire features in terms of traction capability and resistance to aquaplaning, but the rate at which it takes place is an essential qualitative element in judging a tire, since a slow rate of wear corresponds to a long duration of the tire, which is a desired requirement.
On planning a new tire as well as on carrying out quality control of an existing tire, it is therefore of the greatest importance to verify the type and rate of wearing of the tread band.
The verification method mainly used at the present time consists in making a vehicle, provided with test tires, driving over a given road circuit, bringing the tire to its complete wearing or in any event until an important reduction in the tread band thickness occurs; in this manner it is possible to progressively evaluate, by means of inspections at subsequent times, whether the tire wearing is of an even or uneven type and, at the end of the service life of the tire, the rate of wear of the tread.
Unfortunately, for this type of test very long execution times are required in that only abrasions of some importance can be seen with the naked eye and can be measured using the present measurement means and methods which substantially enable wear to be measured through the measurement of the decreased depth of the grooves and the loss of weight suffered by the tire.
Therefore, the above are very demanding and expensive tests, due to the fact that it is necessary to make use of several persons and related vehicles over a long period of time; in this connection, it is important to remember that evaluations are always made by comparison with sample tires.
For example, the tread of a tire may come to the end of its service life after it has covered a distance of about 50,000 km in the case of usual car tires and a distance as high as 150,000 km in the case of heavy duty tires.
Practically, if in a very simple manner a distance of 1,000 km per day is assumed to be covered, the conclusion is that for each type of tire to be tested a person and a test means need to be used over a period of time varying from two to five months.
As an alternative solution to road tests, attempts have been made to standardize the so-called xe2x80x9cindoorxe2x80x9d wear tests, that is tests performed in the laboratory, in workshops or in any case in suitably equipped premises, where the tire is set in rotation against a wheel of a much bigger diameter than that of the tire and covered with a layer of material having predetermined abrasive features.
However, as well known to those skilled in the art, the results of indoor tests can be scarcely correlated with those of road tests; in particular, they greatly depend on the selected abrasive layer and the test modalities, and the abrasion layer features and test conditions do not always succeed in correctly simulating the abrasions caused to the tire during the actual road use. In conclusion, said indoor tests are sometimes trustworthy but sometimes not very reliable, so that they substantially do not seem to have the capability of foreseeing the actual behavior of the tire in use.
The Applicants believe that if it were possible to highlight wear of a small amount in the tread band it would also be possible to carry out road tests of a greatly reduced duration and well capable of foreseeing the final results.
Accordingly, in one aspect, the invention relates to a method of foreseeing and/or controlling wearing of a tread band of a pneumatic tire for vehicle wheels, including the steps of:
a) making a plurality of radial cuts in at least one portion of the radially external surface of said tread band, the depth, width and mutual distance between said cuts being such that, while the tire is being run, a mobility (a movement) is not caused in the portion of said band between pairs of adjacent cuts relative to the corresponding radially more internal band portion which is not cut;
b) rotating the tire at a predetermined speed and with a predetermined load in contact with a friction surface;
c) stopping the tire;
d) verifying wearing of said tread band portion through observation of the maintenance conditions of said cuts.
This result is preferably achieved by keeping the width of said cuts to a value that does not exceed 0.2 mm, and preferably not exceeding 0.1 mm, with the depth of said cuts not exceeding 0.5 mm and the distance between said cuts being at least 2 mm.
Preferably according to this method, steps b), c), d) are provided to be repeated until the cuts have completely disappeared, due to abrasion of the tread portions between each pair of adjacent cuts.
In a preferred embodiment, the method is characterized in that in said tread band portion, cuts are made of a depth between 0.1 and 0.4 mm and most preferably between 0.1 and 0.2 mm.
In accordance with additional aspects of carrying the method of the invention into effect, said cuts are all parallel to each other, inclined to the circumferential direction of the tire and axially extended from one extremity to the other of said tread band or, alternatively, are all parallel to each other and to the equatorial plane of the tire.
The method further provides for said cuts to be distributed into at least two groups of cuts respectively crossing each other and, preferably, for said cuts to be distributed into at least two groups of cuts of different depth.
Conveniently, said groups of cuts of different depth can be formed in different portions of said tread band surface, one portion for each group, or in the same portion, alternated with each other.
Advantageously, the method disclosed herein enables the wear rate of the tread band to be foreseen and/or controlled, preferably according to the steps of:
e) detecting the successive times necessary for the complete disappearance of the cuts of different depth in each group;
f) calculating the wear rate of the tread band depending on said successive times of abrasion of the tread portion confined between the cuts of each group, involving the consequent disappearance of said cuts.
In a different aspect, the invention relates to a method of making said cuts in the tread band by using laser technology or, alternatively, of making the cuts by means of a mold, that is by creating ridges corresponding to said cuts a the mold surface facing the radially external surface of said tread band, using photoengraving techniques for example.
In a further aspect, the invention relates to a pneumatic tire for vehicle wheels comprising a carcass of toric form with a crown and a tread band located on said crown characterized in that on at least one circumferential portions of the radially external surface of said tread band a plurality of radial cuts is formed, the depth, width and mutual distance of said cuts being such that, while the tire is being run, a mobility or movement is not caused in the portion of said band between pairs of adjacent cuts relative to the corresponding radially more internal band portion which is not cut; preferably this result is achieved by keeping the width of said cuts to a value not exceeding 0.2 mm, and preferably not exceeding 0.1 mm, the depth of said cut to values not exceeding 0.5 mm and the distance between said cuts to values at least as high as 2 mm.
In a more specific embodiment, the invention relates to a tire the tread band which is molded with a pattern comprising a plurality of reliefs defined by a corresponding plurality of grooves formed in the band thickness which is provided, on at least one relief, with radial cuts of a depth between 0.3% and 3% of the height of said relief.
Preferably said cuts are formed in at least three distinct portions of the radially external surface of the tread band, circumferentially offset relative to each other; most preferentially said three portions are circumferentially offset by 120xc2x0.
In accordance with different preferred alternative solutions, said cuts have a depth of between 0.1 and 0.4 mm, are all parallel to each other, inclined to the circumferential direction of the tire and axially extended from one extremity to the other of said tread band.
In a preferred embodiment, said cuts are distributed into at least two groups of cuts respectively crossing each other, oriented symmetrically to the equatorial plane of the tire, and define rhombs having diagonals of different length on the tread band, the major diagonal being parallel to the direction of the equatorial plane.
Preferably, said cuts are divided into at least two groups of cuts of different depth, distributed on different portions of said surface of the tread band, one for each group, or alternatively also on the same portion of said surface, alternated with each other.
In an alternative solution, said cuts are all parallel to each other at a zero angle to the direction of the equatorial plane of the tire.
Advantageously, all the above embodiments enable minimum abrasions of the tread band to be verified in an immediate manner.
In a different aspect, the invention relates to a vulcanization mold for tires in which, on at least one portion of the mold""s inner surface facing the corresponding radially external surface of the tire tread band, is provided with ridges of a height not exceeding 0.5 mm.
In a further different aspect, the invention also relates to a method of comparing the behavioral features of the suspension systems of at least two different vehicles equipped with substantially equivalent tires in terms of sizes and performance, through the inspection of the different type and degree of wearing suffered by the treads of identical and identically cut tires mounted to said vehicles.
Preferably the method comprises the operating steps of:
a) mounting identical tires in terms of structure, constituent materials and tread pattern and provided with identical cuts formed according to the method of the invention, at least to the same extremity of the corresponding axle of each vehicle;
b) making the vehicles run on an identical path under the same running conditions;
c) stopping each vehicle after the same running time;
d) inspecting the maintenance conditions of the cuts in said tires and comparing them with each other so as to establish the behavior of the suspension system of each vehicle and, by direct comparison, the spring system offering the best performance as regards tire wear.
Obviously, steps b-d can be repeated several times, until said cuts have completely disappeared so that a sure evaluation of the differences existing between the spring systems under examination is achieved.